Spectrometers are used widely for analyzing optical radiation. A field that has become especially interesting recently is optical coherence tomography (OCT) in the form of the spectral domain OCT, which is also known as Fourier domain OCT. In this case, the spectrometer spectrally disperses an optical interference signal which originates from the optical coherence tomography and records a spectrum. The OCT image is then obtained by means of a Fourier transformation of the recorded spectrum. The advantage of this OCT technology is the very rapid obtainment of the images and an improved signal-to-noise ratio as compared with other OCT technologies.
The requirements placed on the spectrometer in such application are comparatively high because the spectrometer must show high efficiency in combination with high resolution at the same time. The respectively required precision of mechanics and optics leads to considerable complexity. For example, the detector in the spectrometer needs to be adjusted precisely in the micrometer range. It is not surprising that such spectrometers are very sensitive devices and the long-term stability of the adjustment represents a considerable problem in the construction and production. This problem is exacerbated even further in such a way that such devices need to work in certain applications in a normal clinic or office environment, e.g. during opthalmological examinations, which means they need to be relatively sturdy as compared with other highly precise optical appliances.
The constructional compensation of thermal effects for example in spectrometers for OCT apparatuses requires a high amount of technical complexity in order to ensure the desired positional stability of less than 10 micrometers in beam paths of often more than half a meter in length. This complexity is expressed for example in narrow tolerances for the play of screws and fits, high demands on stability for fixing and gluing the detector, and high demands made on the used materials (specific coefficients of expansion, low ageing or fatigue effects). Moreover, large mechanical pretensions are required for adjustable connections in order to minimize the effects of external mechanical forces which may be the result of transport or a changed installation of the spectrometer.
WO 2007/084750 therefore proposes a spectrometer of the kind mentioned above in which an adjusting element is provided which causes the adjustment of the incidence position of the spectrally fanned radiation to the detector in an actively controlled manner. The spectrometer described in WO 2007/084750 uses one property of the detector line used in the spectrometer there for triggering the adjusting element. In this detector line, one portion of the pixels is offset upwardly over the center of the detector line, and another portion downwardly. By averaging all pixels and comparing the added signal intensity of all upwardly offset pixels with the added signal intensity of all downwardly offset pixels, the spectrometer outlined in the mentioned WO publication generates a triggering signal for the actuating element with the goal of centering the incidence position of the fan on the pixel line transversally to the line direction.
The approach of the WO publication is disadvantageous in the respect that the existence of a group of pixels which is offset upwardly in relation to the center of the pixel line will inevitably lead to the consequence that in the case of a correctly adjusted incidence position of the spectrum on the pixel line either a portion of the pixel surface will not be irradiated (which are the upwardly or downwardly protruding sections of the pixels) or a part of the radiation will not meet the pixels despite correct adjustment and cannot be sensed.
WO 2004/043245 A1 describes a spectrometer for an optical coherence tomograph which comprises a two-dimensional detector which comprises several pixel lines. The pixel lines are illuminated successively by means of a scanning device with spectral radiation in order to have more time for reading out the individual lines. The lines of the two-dimensional detector are therefore used as a kind of image storage means.